Photographic support materials consist of film, or of paper, or of coated paper. Film supports are preferred when a high surface quality is demanded, or the flexibility of the material and the cost play a subordinate role, or when an image is to be viewed or used in transmitted light. This is especially true for negative materials, which in general have a lesser thickness and must be transparent.
Positive materials are for reasons of use frequently thicker and are viewed in incident light. They are therefore based preferably on paper, which is still sufficiently flexible, even at a greater thickness, and which possesses the necessary opacity for the viewing of an image in incident light. Photographic papers normally carry a white pigmented covering coating, the composition of which is decisive for the image definition of the photographic image produced in the overlying photosensitive coating.
Rapid and automated processing procedures for the production of positive photographic images utilize to an increasing degree waterproof coated papers. In particular, papers prepared according to German Pat. No. 14 47 815 having both sides coated with extruded synthetic resin serve as supports for a great variety of photographic coatings. The synthetic resin coating disposed between the paper and the overlying photographic coatings contains a light-reflecting white pigment, for example titanium dioxide, and may also contain graduating dyes (shading dyes) and/or optical brighteners and/or other functional additives such as lubricants, antistatic materials, age-resistant agents, etc.
Preferred synthetic resin coatings used are polyolefins, the particular advantage of which consists in the fact that they can be effectively prepared by a simple corona treatment bonding the photographic coatings. Moreover, polyolefin coatings are photochemically inert and economical to produce.
A disadvantage of all support materials on a paper base produced by extrusion coating is the relatively low content of white pigment in the pigmented covering layer. Polyolefinic extrusion coating materials contain not more than about 20% by weight titanium dioxide, because at higher pigment contents it is not possible to produce a cohesive film coating.
A further disadvantage of extrusion-coated support materials arises from the high temperature required to extrude the resin which is deposited on the paper in the molten state at approximately 300.degree. C. Extrusion of polyolefin coatings on a paper support is suggested in U.S. Pat. No. 4,283,486, which patent also suggests coating from a solvent solution of the polymer and drying by hot air. As a consequence of the sudden heating-up of the paper surface by either of these methods, a dehydration of the cellulose fibers situated at the surface takes place. This sudden dehydration and the partial reconditioning of the fibers that sets in after cooling are accompanied by an undesired, finely-dented deformation of the paper surface, which is reflected in the overlying synthetic resin coating. Consequently, melt-coated photographic paper supports always exhibit a surface structure which may be described as fine uneveness, which manifests itself in the photographic image as "mottle", that is a cloudy turbulence.
A further cause of uneveness on a synthetic resin coating produced on paper by extrusion coating from the melt with subsequent cooling at a roll surface is attributable to the non-homogeneity of the coated paper core. Corresponding to the density fluctuations in the paper, a variably-strong bond of the synthetic resin coating to the cooling roll is produced, and when the coated paper is separated from the cooling roll deformation forces of varying strength act upon the surface of the still plastic synthetic resin coating.
The disadvantages in regard to surface structure and image definition of such papers coated with thermoplastic resin coatings can be overcome according to co-pending U.S. applications Ser. No. 273,110, now U.S. Pat. No. 4,364,971 and Ser. No. 273,111, now U.S. Pat. No. 4,384,040 by coating the base paper with hardenable resin mixtures, the hardening of which is carried out using electron rays at room temperature. Waterproof photographic paper supports produced in such manner give rise, on account of increased pigmentation, to photographic images of good image definition. Further they do not have the uneven surface which copies the paper structure and which can manifest itself in the finished photographic image as "mottle".
A disadvantage of such support materials having one or more coatings of mixtures hardened by electron ray is, however, that a small proportion of monomeric compounds always remains behind in the coating. These may be monomers that were not cured during the electron radiation hardening, or depolymerization products. It is believed that other reactions take place upon exposure to the radiation which generate radicals that form peroxides, or acrylic acid which is reduced to acrolein. It is possible also that destructive particles can be generated from the sizing on the base paper. The proportion of these mostly volatile, but in any case mobile chemical compounds of different structure, is relatively low, but they will migrate to the photosensitive emulsion. On account of their chemical reactivity, these compounds can adversely affect the photographic coatings and in particular vary their sensitivity during storage. Even usual bond-promoting coatings on a base of gelatins, or mixtures of gelatins with other polymers, have not proved to be adequate barriers for preventing migration of these harmful reactive compounds. Even a subsequently applied coating of polyethylene foil did not prevent migration and the consequent adverse effect on the overlying photographic layer.